I would speak to them as BT lowest tarrif is 10GB per month. Not sure if they charge per byte after that limit or just cut you off.
If thats the case ask to upgrade for a few quid extra a month.
I bet the cost is down to phone calls to mobiles though – any family been abroad over summer etc?
Taken from BT – here
If you exceed your monthly usage allowance your service won't stop working; advisory emails will be sent to your BT primary email address if you reach 80 per cent of your usage allowance each month and further notification if you then exceed your allowance in a particular month.
If you exceed your usage allowance for two months in a row, we'll charge you £1 for each extra GB you use (rounded up to the nearest GB) starting from the second month you exceed your allowance. We'll continue charging if you keep exceeding your allowance in the following months. These charges will appear on your BT bill.
From 1st November 2010, if you exceed your usage allowance, you'll be charged for additional usage in units of up to 5 gigabytes (GB), at a charge of £5 per 5GB. Charges will apply from the second month you exceed your allowance and in any subsequent month that the usage allowance is exceeded. These charges will appear on your BT bill. To contact us about these changes visit